Mercury switch



Nov. 1,7, 1936. f c. H. LARsoN 2,060,810

MERCURY SWITCH original Filed oci. 5, I1932 2 sheets-sheet 1 mlm.

z iii i Si INVENTOR.

A TTORNEYSI Nov. 17, 1936. c, H, LARSON' 2,060,810

lMERCURY SWITCH Original Filed Oot. 5, 1932 2 Sheets-Sheva'l 2 Mw sm" l N VEN TOR.

A TTORNEYS,

atenteci Nov. l?,

urns

PAT

Application @ctoher 5, 1932, Serial No., 636,288 Renewed April tu, letti si calma (ci. stomme) The principal objec vide a mercury switch place ci the mechanical re railway signaling systems: illustrated and described Signaling Principles an lu, published by American Railwa s invention is to pro tei relays being mexican Railway ticos chapters' d 311i Vesey St., New ever, of the relay is obviously not limited to r lay signaling systems.

Further and other objects and advantages will become apparent as the disclosure proceeds and the description is read in conjunction with the ccompanying drawings, which illustrate two preierred embodiments of invention.

In the drawings:

Fig. l is a vertical, sectional view oi the pulldown embodiment of this invention, the plunger being shown in the position which it assumes when the associated electromagnet is deenergized;

Fig. 2 illustrates the functioning of the relay by indicating the relationship of the plunger, mercury and contacts shortly after the electromagnet has been energized, but prior to the plunger reaching its maximum depressed position;

Fig. 3 shows the plunger in its maximum depressed position. l

Fig. 4 is a perspective view of the plunger with illustrative specific dimensions given;

Fig. 5 is a plan view of the relay;

Fig. 6 is a circuit diagram illustrating one environment in which the relay may be used;

Figs. 7 through l2 inclusive corresponds to Figs. l6 inclusive, respectively, the only difference being that the relay is of the pull-up type.

The illustration and description of the preferred forms of the invention are for the purpose of disclosure only and are not to be construedv as imposing any limitation on the appended claims.

It has already been demonstrated in practice that mercury switch relays have certain decided advantages over the mechanical relays heretofore commonly used in railway` sigjnalin'g systems. The fact that all electrical contacts of a mercury switch relay are hermetically sealed within a glass envelope and in consequence are not subject to the deteriorating effects of the atmosphere'is but one of the cardinal advantages of this type of relay.

Otheradvantages which may be briefly mentioned reside in the elimination of bearings and movable parts; the constant and'extremely low contact' resistance unaffected by temperature changes of 40 below zero to 300 above zero; the lack of wear on the contact material; the noiseless operation on both alternating current and direct current; the economy o space due to compact mounting; and the'completeelirninatlon of the necessity for inspection and maintenance.

The mercury switch relay o this invention is designed to offer greater economies in the cost of Y operation and manufacture and to provide greater reliability in use.

Referring now to the drawingsY and particularly to Figs. l. to 6 inclusive, there is illustrated a preferred embodiment oi' the invention in which.

the electromagnetcally actuated plunger is of the pull-down type. In other words, when the relay lis ile-energized, the plunger floats on the mercury and establishes electrical contact between two electrodes and when the relay is energized, the plunger is drawn downwardly to uncover one oi those electrodes and establish electrical connection between the other electrode and a third electrode'.

The relay comprises a glass envelope 2li, evacuated and lled with a suitable inert gas such as helium or hydrogen, after which it is sealed off at 2l. Three electrodes 22, 23 and 2B are pinch sealed, as shown at 25, into the base of the envelope and project to varying heights into the interior of the envelope.

The envelope 20 may be made of any suitable glass, low expansion boro-silicate glass or glass of similar quality being preferred.. The electrodes 22, 23 and 24 may be tungsten. molybdenum, copper-coated nickle alloy wire, or any other suitable material that will form a suitable seal with the glass envelope.

A quantity of mercury 26 is sealed' into the switch envelope and is in constant electrical contact with the electrode 22. The electrode 23 is insulated from the mercury by a glass sleeve 21 to a height less than the height of the electrode 23 so as to leave a portion of the electrode 28 exposed to the mercury in the envelope. The electrode 24 is similarly insulated by a glass sleeve 29 leaving` a portion 30 of the electrode bare. In the particular switch illustrated, the electrodes 22, 23 and 24 rise to heights of 1%", 2H" and 3%", respectively, and a sulcient quantity of mercury is used'to cover the electrode 23 when the relay is de-energized.`

A plunger or displacer 3| placed in the envelope before it is evacuated, lled with an inert gas and sealed off, is adapted to reciprocate within the envelope under the lnuence o! an electromagnet, generally indicated at Il, for the purpose of changing the mercury level and selecting which one of the electrodes 23 and 24 shall be in electrical contact with the electrode 22. The A plunger 3| consists ci! a sleeve 32 of magnetic material, such as Swedish iron, although it may be made of any suitable magnetic material. The sleeve 32 has an external diameter slightly less than the internal diameter of the envelope, this diierence being substantially bridged by lugs 33 on the top and bottom of the plunger to guide the plunger in its vertical movement. Springs 34 and 35 are provided at the top and bottom, respectively, of the plunger to protect the envelope from injury due to rough handling. The upper spring, preferably, limits the upward movement of the plunger to insure uniform performance oi the relay but having an excess of buoyancy, the plunger will always rise to its proper position, and in addition the pick-up and drop-away voltages are brought closer together. The top of the plunger 3| is rounded oil at 36 and provided with a socket 31 into which an inverted cup-shaped element or cap 38 is adapted to be adjustably seated. The adjustability of the cap 38 permits correction to be made for manufacturing tolerances. so that the relay will have the required characteristics. It is not essential that the can 3B be ol magnetic material, but it is preferred.

The cap being concentrically mounted with respect to the sleeve 32 is directly over the elec- Erode 23 which is located on the axis of the switch. The electrode 24 ts in a slot 39 in the wall oi. the sleeve 32, the slot extending from the socket 31 at the top of the plunger down to the base of the plunger. This slot in addition to furnishing a place into which the electrode 24 may iit, and permitting the mercury to rise freely on the outside of the cap without entrapping gas, also increases the efiiciency of the relay about 40% when the latter is operated on alternating cur rent. This is apparently explained by the fact that the slot prevents the ilow of induced currents in the plunger resulting from the constantly changing direction oi the magnetic field. The slot also permits a compact arrangement of the switch parts within the envelope.

The electromagnet which operates the relay consists of a laminated yoke 40 between the arms of which a spool 4| with its winding 42 is mounted. Within the spool 4| and surrounding the switch envelope are pole sleeves 43 and 44 .suitably secured to the laminated yoke 4D and made o! ferro-magnetic material. The air gap indicated at 45 which separates the pole pieces is bridged by the plunger 3| when the electromagnet is energized and the plunger thus acts as an ai mature of the magnet.

In operation, when the electromagnet is deenergized, the plunger 3| floats upon the mercury and assumes the position shown in Fig. l. In this position, the electrode 23 is covered by mercury due to the relationship oi the height of the electrode, the displacing eiect of the plunger, and

the quantity of mercury within the tube.

When the relay is energized, the plunger sinks in the mercury in its etl'ort to close the air gap between. the pole pieces 43 and 44 and iirst uncovers the electrode 23, as shown in Fig. 2. The

cap 3l, as will be seen, forms a gas trap which forces the mercury away from the bared portion 2l of the electrode and completely lbreaks the electrical circuit between the electrode 22 and the electrode 23 before the mercury level reaches a height on the outside o! the cap suillclent to establish electrical connection between the electrode 24 and the electrode 22. This occurs, however, upon further depression of the plunger, as will be seen trom Fig. 3. The cap 33 being made of impervious material holds the mercury away from the central electrode as long as the plunger is lowered.

When the relay is used to transfer a load from one source of current to another, the electrode 22 is connected to the load and the electrodes 23 and 24 are connected to one side oi' the two sources, respectively. This is best illustrated in Fig. 6 which is a circuit diagram showing a railway signal 46 normally operated on alternating current but adapted to be operated on direct current whenever the former falls below a predetermined standard.

The alternating current is usually supplied from a power line and is reduced in voltage by a transformer 41, the secondary coil of which is in series with the signal. A local battery 4B is provided at suitable points along the line so that upon failure of the alternating current, the signal will be operated from the local circuit.

To better illustrate the use of the relay in such a circuit, the relay 49 in Fig. 6 has been shown as a mechanical relay, but it will be understood that the coil 5U corresponds to the coil 42 of the mercury switch relay; the armature 5| to the plunger 3| the conductor 52, which connects the armature to the signal 46, corresponds to the electrode 22; the contact 53, to the electrode 23; and the contact 54 to the electrode 24. The spring 55 causes the armature 5| to contact with the contact 53 when the coil 5I] is tiel-energized and, therefore, corresponds to the buoyancy of the mercury which maintains the plunger 3| in elevated position when the coil 42 is deenergized. Thus, whenever the voltage in the secondary coil cf the transformer 41 falls below a predetermined standard, the spring 55 in the mechanical relay, Fig. 6, (or the buoyancy of the mercury in the mercury switch relay, Figs. 13) causes the alternating current circuit to be brokenat 54 (the electrode 23 Figs. l-3) and the direct current circuit to be made at 53 (the electrode 24, Figs. 1 3). If later the alternating current picks up to its predetermined standard, the armature 5| is pulled away from the contact 54 and the alternating current circuit is reestablished.

In Figs. 712, a second preferred embodiment of the invention is shown. This embodiment differs from the relay just described only in that it is of the pull-up type, i. e. the plunger is pulled out of the mercury when the relay is energised instead of being lowered into the mer cury as was the case of the relay shown in Figs. l-6 inclusive. Due to the diierence in the mode of operation of the two types of relays, the position of the air gap, the length of the electrodes and the dimensions of the plunger are somewhat different. The operation of the switch, however, is substantially the same.

As seen in Figs. 79 inclusive, the pole sleeves 56 and 51 provide an air gap 58 adjacent the top of the switch envelope which is adapted to be closed by the plunger 59 when the coil 50 is energized. The plunger 53, except Ior its dimensions, ls identical in every respect to the plunger 3| shown in Figs. 1 4 inclusive, and, therefore, need not be specifically described.

The electrodes 6|, $2 and 33 correspond to the electrodes 22, 23 and 24, respectively, of the "pulldown relay and, in the switch shown, rise to heights of 1% and 1H", respectively, within the envelope. A sufficient quantity of mercury 64 is placed in the envelope to cover the electrode 33 when the switch is deenergized.

It is apparent from Figs. 7 to 9 inclusive that trode tl while still maintaining the electrode t2 out of Contact with the mercury. As the plunger rises higher in the envelope, the cap is lifted above the top of the electrode and allows mercury to come in contact with the electrode to establish an electrical connection with the electrocle di.

The operation or the switch is diagrainmatically illustrated in Fig. l2 which is similar in all respects to the diagram of liig. 6 with the exception that the armature 5l in this case alls hy gravity on the contactl 53 when the alternating current fails instead of being raised by a spring d5 (llig. 6) or the buoyancy of the mercury (Figs. l-3) as in the pull-down type of relay.

ln addition to the speciiic data which have already been given with respect to the height oi the electrodes, the quantity oi mercury, etc. which, it will be understood is for the purpose ci disclosure only, speciilc dimensions oi the piling ers til and have heen. indicated in Figs. and lil, respectively.

'in both embodiments of the invention, the distance between the pole pieces is and the switches are located in a position so that the plunger in the pull-down switch is /a" above the lower pole piece ln the deenergized position and on the pull-up switch, the plunger is l/a" above the upper pole piece the deenergized position. The coils for operating the relays are wound with l5 gauge enameled wire, 26@ turns, drawing 2 watts at lll volts. The pick up with this rating oi coil is 7 volts ior the pulldown type and 4.5i volts for the pull-up type, which `indi= cates that the pull-up type approximately 30% more efficient.

ln both embodiments oi the invention, it will be noted that the cap gives additional buoyancy to the plunger by reason of the gas trap that it forms when in its lowered position. By this cori1 struction, more of the mass oi the plunger may be concentrated in the outer sleeve where operating efficiency is increased by lessening the air gap lietween the armature and the pole pieces.` Approximately 30% is gained in efficiencyby constructing the plunger in this way.

It will be understood that many oi the advanm tages of this invention may be utilized in a two electrode switch by eliminating one or the other of the intermittently covered electrodes.

To protect the plunger cap against the deteriorating effects of arcing when the switch is used with high amper-age, the interior of the cap may be lined with a ceramic sleeve in a manner Well mersed in several of the claims is intended to include immersion of the electrodes at either regular or irregular intervals of time.

What I claim, therefore, is:

l. In an electro-magnetic mercury switch, the combination or a vertical switch envelope having a quantity of mercury therein, a plurality oi' electrodes sealed through the envelope, one being constantly in contact with the mercury and two loeing alternately, but never simultaneously, in contact therewith, a plunger floating on the mercury and determining, by the amount of mercury that it displaces which ot the two intermittently immersed electrodes is in contact with the mercury, electromagnetic means ior changing the amount oi mercury displaced by the plunger, said plunger being in the form of a sleeve provided with an inner cap adapted to t over one oi the two intermittently covered electrodes when the plunger is in its lowered position and cause the electrode to loe uncovered.

2. in an electromagnetic mercury switch, the combination of a vertical switch envelope having a quantity of mercury therein, a plurality of electrodes sealed through the envelope, one being constantly in contact with the mercury and two loeing alternately, hut never simultaneously, in contact therewith, a plunger 'floating on the mercury and determining, by the amount oi mercury that it displaces which of the two intermittently immersed electrodes is in contact with the mercury, electromagnetic means 'for changing the amount of mercury displaced hy the plunger, said plunger comprising a sleeve provided with a pocket arranged above one of the two intermit` tently covered electrodes and adapted, when the plunger is lowered, to trap gas within the cap and force the mercury trom the electrode.

3. ln an electromagnetic mercury switch, the combination of a vertical switch envelope having a quantity ci mercury therein, a plurality of electrodes 4sealed through the envelope, one heing constantly in contact with the mercury and two being alternately, but never simultaneously, in contact therewith, a plunger floating on the mern cury and determining, hy the amount oi" mercury that it displaces which oi the two intermittently immersed electrodes is in contact with the mercury, electro-magnetic means for changing the amount oi mercury displaced by the plunger, said plunger comprising a cylindrical sleeve having a smaller sleeve closed at the top, carried interiorly of the first named sleeve, and adapted to force mercury from one oi the two intermittently covered electrodes when in its lowered position.

li. in an electromagnetic mercury switch, the combination of a vertical switch envelope having a quantity of mercury therein, a plurality oi electrodes sealed through the envelope, one being` constantly in contact withthe mercury and two being alternately, but never simultaneously, in contact therewith, an elcctromagnetically actuu at'd plunger floating on the mercury and adapted to control the level o f the mercury in the envelope, said plunger comprising a sleeve of magnetic material having an external diameter slightly less than the internal diameter of the envelope, and carrying an inverted cup-shaped element adapted to form a gas trap over one of the intermittently immersed electrodes when the plunger is in its lowered position to thereby uncover that electrode.

5. In an electromagnetic mercury switch, thc combination of a vertical switch envelope having a quantity of mercury therein, a plurality of eleclil trodes sealed through the envelope, one being constantly in contact with the mercury and two being alternately, but never simultaneously, in contact therewith, an electromagnetically actuated plunger floating on the mercury and adapted to control the level of the mercury in the envelope, said plunger comprising a sleeve of magnetic material and being equipped with a pocket, closed at the top, for creating a gas trap over one of the intermittently covered electrodes when the plunger is in its lowest operative position, thereby uncovering the one electrode and establishing electrical connection with the other.

6. In a mercury switch. a switch envelope, a quantity oi mercury therein, a pair of electrodes sealed through the envelope, and a plunger for making and breaking an electrical circuit through the electrodes, said plunger having an inner,

i downwardly projecting impervious cap adapted to telescope over one of the electrodes and hold mrrcury from said electrode when the plunger is in lowered position.

7. In a mercury switch, a switch envelope, a quantity of mercury therein, a pair of electrodes sealed through the envelope, and a plunger for making and breaking an electrical circuit through the electrodes, said plunger having an inner, downwardly projecting impervious cap adapted to increase the buoyancy of the plunger when depressed.

8. In a mercury switch, a vertically supported switch envelope, a quantity of mercury therein, a pair of electrodes sealed through the envelope and adapted to complete an electrical circuit when bridged by the mercury, an electromagnetically actuated plunger in the envelope for manipulating the mercury to change the condition of the circuit, said plunger comprising an outer sleeve, the external diameter of which is slightly less than the internal diameter of the envelope, and an inner sleeve, closed at the top, adapted to form an impervious gas trap over one oi' the electrodes when the plunger is in its lowered position to thereby hold mercury from the electrode and maintain the electrical circuit open.

9. A plunger for use in a switch o1' the class described comprising an outer sleeve of substantially uniform external diameter, and an inner sleeve, closed at the top, and impervious, mounted in said first named sleeve.

10. A plunger for use in a switch of the class described comprising a sleeve of magnetic material having a substantially impervious inverted cup-shaped sleeve interiorly mounted in the first named sleeve.

l1. A plunger for use in a switch of the class described comprising a sleeve of magnetic material having an inverted cup-shaped sleeve interiorly mounted in the first knamed sleeve and longitudinally adjustable relative to said sleeve.

12. A plunger for use in a switch of the class described comprising an outer sleeve of magnetic material having a uniform external diameter slightly less than the internal diameter of the switch envelope, guide lugs on the sleeve for maintaining it at the appropriate distance from the envelope, and an inverted cup-shaped sleeve of impervious material interiorly mounted in the first named sleeve.

13. A plunger for use on a switch of the class described comprising a sleeve of magnetic material having an inverted cup-shaped sleeve of impervious material interiorly mounted in the first named sleeve, said outer sleeve having a longitudinal slot extending to a point above the bottom of the cup-shaped inner sleeve.

14. A plunger for use in a mercury switch comprising a cylindrical sleeve of magnetic material closed at its upper end except for a socket opening and a slit extending from one side of the opening down to the bottom of the sleeve, and an inverted cup-shaped member adJustably seated in said socket opening.

15. In a mercury switch, a switch envelope, a quantity of mercury therein, a pair of electrodes sealed through the envelope, and a plunger for making and breaking a circuit through the electrodes, said plunger comprising an armature of magnetic material and an inverted cup-shaped member of impervious material adapted to dlp in the mercury.

16. A plunger for use in a mercury switch comprising an armature of magnetic material and an inverted cup-shaped element of impervious material carried by the armature to increase the buoyancy of the plunger.

17. In a mercury switch, a switch envelope, a quantity oi.' mercury therein, a pair oi electrodes sealed through the envelope, and a plunger for making and breaking a circuit through the electrodes, said plunger comprising an armature ofl magnetic material, and being equipped with means ior creating an impervious gas trap over one of the electrodes to hold mercury from that electrode when the plunger is lowered.

18. A plunger for use in a mercury switch, having a portion made of magnetic material for responding to a magnetic field and a portion adapted to form an impervious gas trap when closed at the bottom by mercury.

19. In a mercury switch, a switch envelope, a quantity of mercury therein, a pair of electrodes sealed through the envelope, and a plunger for making and breaking a circuit through the electrodes, said plunger comprising an armature of magnetic material and including an inverted cupshaped member of impervious material adapted to telescope over one of the electrodes to force mercury from that electrode when the plunger is lowered.

20. In an electromagnetic switch, the combination of a vertical switch envelope having a quantity of mercury therein, a plurality of electrodes sealed through the envelope, one being constantly in contact with the mercury and two being alternately, but never simultaneously, in contact therewith, a plunger floating on the mercury and determining by the amount of mercury that it displaces which of the two intermittently immersed electrodes is in contact with the mercury, electromagnetic means for changing the amount of mercury displaced by the plunger, said plunger including means for forcing mercury from one intermittently immersed electrode when the other of said electrodes is in contact with the mercury.

21. A standby relay for transferring a load from a primary source of electrical energy to an auxiliary source whenever the former falls below a pre-determined value, said relay comprising an electromagnetic switch having front and back contacts hermetically sealed fromthe atmosphere and adapted to be connected one to each bf said sources, mercury associated with said contacts, and means controlled by the electromagnet for selecting one of the contacts to be placed in direct physical contact with the mercury.

22. In a mercury switch of the class described, a vertical switch envelope, a quantity of mercury therein, spaced electrodes in the envelope, two

aoeasio being at different heights and one oi the two being normally in contact with the mercury, and means for bringing the other of the two electrodes into contact with the mercury and substantially simultaneously uncovering the first, said means cluding a displacer for shifting the mercury level, and means ior forming a gas trap over the drstmentioned electrode.

23. in a mercury switch ci the class described, a vertical switch envelope, a quantity oi mercury therein, spaced electrodes in the envelope, two being at different heights and one of the two being normally in Contact with the mercury, and means including a mercury displacer :for forcing mercury from the normally covered electrode and causing mercury to contact with the other oi two electrodes.

2e. in a mercury switch of the class described, a vertical switch envelope, a quantity of mercury therein, spaced electrodes in the envelope, one being normally in contact with the mercury and another being normally out of contact with the mercury, and means for reversing the relation of the said electrodes to the mercury, said means including a displacer equipped with means ior forming a gas trap over one of said electrodes when the displacer is in its lowered position.

25. ln a mercury switch oi the class described, a vertical switch envelope, a quantity of mercury therein, spaced electrodes in the envelope, one being normally in contact with-the mercury and another being normally out of contact with the mercury, and means for reversing the relation ci the said electrodes to the mercury, said means including a displacer equipped with means ior forming a gas trap over one of said electrodes when the displacer is in its lowered position, said other electrode being outside oi the gas trap and subject to the mercury displacement action or the displacer.

26. in a mercury switch of the class described, a vertical switch envelope, a quantity of mercury therein, a displacer for shifting the mercury, spaced electrodes in the envelope, two being at different heights and positioned so that one is above the mercury level and the other immersed in the mercury when the plunger is in raised position, and means for forcing mercury from the immersed electrode when the displacer is lowered.

27. lin a mercury switch of the class described, a vertical switch envelope, a quantity oi mercury therein, a displacer for shifting the mercury, spaced electrodes in the envelope, two being at different heights and positioned so that one is above the mercury level and the other immersed in the mercury when the plunger is in raised position, and means carried by the displacer for forcing mercury from the immersed electrode when the displacer is lowered.

28. A switch envelope for` use inr a mercury switch of the type in which a displacer is magnetically controlled for manipulating mercury within the envelope,said envelope comprising a tubular body having at least three electrodes sealed through the base thereof, two of the said electrodes projecting upwardly Within the envelope and both being surrounded by insulating sleeves for a portion of their lengths.

29. A switch envelope for use in a mercury switch of the type in which a displacer is magnetically controlled for manipulating mercury Within the envelope, said envelope comprising a tubular body having at least three electrodes sealed through the base thereof, two of the said electrodes projecting upwardly within the envelope and both being surrounded by insulating sleeves ior a portion of their lengths, one of the insulating sleeves being shorter than the other.

Bil. lin an electromagnetic mercury switch, a combination oi a vertical switch envelope having a quantity oi mercury therein, spaced electrodes .in tb envelope, an electro-magnetically actuated plunger ioating on the mercury and adapted to control the level oi the mercury, and a spring above the plunger adapted to normally hold the plunger lower than its normal floating position.

3l. a mercury switch of the class described, a switch envelope, a mercury nil, a plurality of spaced electrodes sealed through the base of the envelope and projecting upwardly, one being substantially along the axis of the envelope, and another being ladjacent to the side wall of the envelope, a displacer telescoped over the central envelope for shifting the mercury level, said displacer having a longitudinal slot receiving the side electrode whereby the overall dimensions of the switch may bekept to a minimum.

32. A standbyrelay for transferring a load from a primary source of electrical energy to an auxiliary source whenever the former ialls below a pre-determined value, said relay comprising an electromagnetic switch having :iront and baclf; contacts hermetically sealed from the atmosphere and adapted to be connected one to each of said sources, mercury associated with said contacts, and means controlled by the electrornagnet for selecting one oi' the contacts to be placed in direct physical contact with the mercury, saicl means including a displacer responsive to the electromagnet for changing the level oi the mercury, v

35i. in a mercury switch, a switch envelope, a quantity oi mercury therein, a pair or electrodes sealed through the envelope, and a plunger for making and breaking the circuit through the elec- A trodes, said plpnger comprising an armature of: magnetic material and including an inverted cupshaped member adapted to telescope ov'er one oi the electrodes to forces mercury from that electrede when the plunger is lowered, the other eleca trode being surrounded by an insulating sleeve for a portion of its length and being responsive to the mercury level outside of the plunger.

3d. in a mercury switch oi the class described, a vertical envelope, a charge of mercury in the envelope, a magnetically controlled displacer for changing the mercury level, a plurality' ci electrodes within the envelope, one of which is responsive to the mercury level on the outside oi the displacer, said displacer having means for forming a gas trap over another of said electrodes to force mercury away from said electrode when the displacer is lowered.

35. A. standby relay for use in railway signalling systems for transferring a load from a primary source of electrical energy to an auxiliary source whenever the former falls below a predetermined value, said relay comprising an electromagnetic switch having independent front and back contacts hermetically sealed within a vertical switch envelope and adapted to be connected one to each of said sources, mercury associated with said contacts, and means controlled by the electromagnet for selecting one of the contacts to be placed in direct physical contact with the mercury, said means including a displacer responsive to the electromagnet for changing the level of the mercury.

36. A standby relay for use in railway signaling systems for transferring a load from the prilil 'Ill

mary source oi electrical energy to an auxiliary source whenever the former falls below a predetermined value, said relay comprising an electro-magnetic switch having independent front and back contacts hermetically sealed within a vertical switch envelope and adapted to be connected one to each of said sources, mercury associated with said contacts, and means controlled by the electromagnet for selecting one of the contacts to be placed in direct physical contact with the mercury, said means including a displacer responsive to the electromagnet for lowering the level of a. portion o! the mercury illl and raising the level of another portion whenever the displacer is moved downwardly.

37. In a mercury switch of the class described, a switch envelope, a mercury fill. a plurality oi' spaced electrodes sealed through the base of the envelope at least one oi' which is adjacent to the side wall of the envelope, a displacer for shifting the mercury level, said displacer having a longitudinal slot receiving said one electrode whereby the overall dimensions o! the switch l0 may be kept to a minimum.

CARL H. LARSON. 

